Many larger aircraft use main landing gear that has two or more sets of wheels mounted on a bogie beam, the bogie beam in turn being pivotally mounted to the main sliding tube/leg of the landing gear. In such arrangements it is common practice to provide a mechanism for controlling the relative positions of the sliding tube and the bogie beam, either to position the landing wheels in a desired attitude prior to actual touchdown, or to position the wheels and bogie beam relative to the sliding tube in a particular manner to facilitate storage of the landing gear in the fully retracted position, or a combination of both. The positioning of the bogie beam is generally accomplished using a pitch trimmer, which is typically a hydraulically operated telescopic actuator, often in combination with one or more mechanical linkages.
The pitch trimmer has the additional function of damping the motion of the bogie beam relative to the sliding tube, both whilst the aircraft is manoeuvring on the ground and also during the retraction, and possibly also the extension, phase of the landing gear. One of the more demanding instances where the pitch trimmer provides damping of the relative motion between the bogie beam and sliding tube is the rotation of the bogie beam relative to the sliding tube that occurs as the landing gear extends during and immediately after actual take off of the aircraft. Typically, the extension of the landing gear as the weight of the aircraft is removed causes the bogie beam to rotate relative to the sliding tube towards a position in which the leading pair of wheels are raised higher than the trailing pair of wheels, this being referred to as the normal trail position of the bogie beam. During this phase, referred to as free recoil, there is a potential for the bogie beam to “over rotate” beyond the normal trail position. Depending upon the rate at which the sliding tube extends, i.e. the sliding tube extension velocity, the bogie beam will have varying rotational energies when it reaches the normal trail position. For rotational energies beyond a typical prescribed limit, over rotation of the bogie beam will occur that may be sufficient to overcome the damping provided by the pitch trimmer and result in contact between the bogie and the sliding tube. This contact is not desired as it may cause damage to the bogie beam and/or the sliding tube.
One existing solution to mitigate the effects of such undesired contact between the bogie beam and sliding tube is to fit one or more stop pads on the bogie beam that provide a degree of separation between the bogie and sliding tube should over rotation occur. However, such stop pads do not actually stop the contact between the stop pad itself and the sliding tube occurring and additionally provide little or no damping of the motion or the contact when it occurs. Consequently, damage to the sliding tube or the stop pad may still occur, whilst the addition of the stop pads introduces undesirable additional overall weight to the landing gear assembly.
Existing pitch trimmers have a degree of damping which damps low energy over rotations to position the bogie in its correct orientation, preventing contact between the sliding tube and bogie beam. This damping is however designed to damp landing loads and bogie motion during four-wheel contact ground movements and consequently the damping is insufficient to arrest the bogie rotation in the increasing trail direction during over rotation of the bogie beam during free recoil.